QUANTIFIERS-USING-RECURSION

The following example illustrates the use of recursion as a means of
avoiding proof difficulties that can arise from the use of explicit
quantification (via defun-sk). See quantifiers for more about
the context of this example.

(in-package "ACL2")

; We prove that if every member A of each of two lists satisfies the
; predicate (P A), then this holds of their append; and, conversely.